Submersible pumping systems are often deployed into wells to recover petroleum fluids from subterranean reservoirs. Typically, a submersible pumping system includes a number of components, including one or more electric motors coupled to one or more pump assemblies. The submersible pumping systems deliver the petroleum fluids from the subterranean reservoir to a storage facility on the surface. Each of the components in a submersible pumping system must be engineered to withstand the inhospitable downhole environment.
Submersible pumping systems can be attached to the end of production tubing or coiled tubing to deliver fluids to the surface. Various configurations of the components in the submersible pumping system may be used based on the type of delivery system and on various well conditions. For example, some submersible pumping systems that use production tubing through which to deliver fluids to the surface employ a pump above the motor (top intake). Conversely, some submersible pumping systems that employ coiled tubing and that utilize well casing to deliver fluids to the surface employ a pump below the motor (bottom intake).
Referring now to FIG. 1, shown therein is a prior art submersible pumping system 200 in a top intake configuration. The submersible pumping system 200 is disposed within a well annulus 202, and includes a motor assembly 204, a seal section 206 and a pump assembly 208. A power cable 210 extends from the surface through the annulus 202 and connects to the motor assembly 204.
During operation a shaft (not shown) in the motor assembly 204 rotates and drives a shaft (not shown) in the seal section 206, which in turn drives the pump assembly 208 to propel well fluid through the production tubing 212.
It is well known that during startup of a submersible pumping system, the motor shaft tends to rise, an effect known as “up thrust.” In top intake applications this problem is diminished by use of the seal section 206 between the motor assembly 204 and the pump assembly 208, which not only facilitates motor lubricating oil expansion and contraction, but also prevents upward movement of the motor shaft. However, in bottom intake applications the seal section is positioned below the motor and therefore is unable to prevent the motor shaft from moving upward during startup. This problem is more pronounced in horizontal wells since the effect of gravity is virtually eliminated from holding down the motor shaft.
The rising motor shaft causes wear on various components of the motor and causes excessive wear on motor bearings. Motors are typically fitted with a radial bearing at the upper end of the motor, and these upper bearings frequently take the brunt of the up thrust generated during startup. Excessive wear on the upper bearings can cause the bearings to fail and can ultimately result in failure of the motor.
It is therefore desirable to control the effects of up thrust in a motor, especially in configurations of submersible pumping systems that are susceptible to excessive wear such as bottom intake systems in deviated wells. It is to these and other deficiencies in the prior art that the present invention is directed.